Carbon occurs in a variety of forms in many refractory and ceramic shapes which are used to either contain molten metals and slags or to provide a shield from intense heat. For example, carbon added to a shape can occur in a range of sizes from additions of sub-micron carbon black to large 1 to 2 mm particles of petroleum coke or flake graphite. Carbon can also occur in the form of pore filling carbon derived from tar or pitch impregnation of a sintered or coked refractory shape. In addition, carbon often occurs as the bonding material in many refractories. This form of carbon is derived from high carbon-yielding resins or from molten tar or pitch. After shape forming, the refractories are heated under reducing conditions at temperatures over 1000.degree. F. This treatment removes the light hydrocarbons from the carbon precursor and leaves a carbon residue which forms a bridging network. This network provides a means of providing strength to the shape.
Carbon has many useful purposes. It is often added to a refractory or ceramic to increase its thermal or electrical conductivity or to provide a nonwetting barrier to the intrusion of molten slag or corrosive gases.
A fundamental problem with any refractory shape which contains carbon is oxidation. Depending on the size of the carbon particle, oxidation can begin as low as 500.degree. F. or less. Most carbon-containing shapes are used at much higher temperatures, often at 3000.degree. F. or higher. As temperature and time increase carbon becomes increasingly vulnerable to gaseous removal by oxidation. Loss of carbon through oxidation limits the service life of the refractory and defeats the intended purpose of adding carbon to the shape. Furnace operators often employ schemes to limit carbon oxidation, such as preheating under reducing conditions, and refractory manufacturers often add fine metals like silicon or aluminum to limit oxidation. These efforts, however, do not entirely prevent oxidation.
Other efforts to overcome this problem have included use of protective coatings, such as alkali silicate, lead silicate, and numerous other materials in order to prevent oxidation of the carbon in the refractory or ceramic-shape articles. These have not proven satisfactory and the refractory shapes, particularly the bricks, still suffer undesirable weight losses when exposed to the conditions to which they are utilized; such as the high temperatures in process furnaces.